1. Field of the Invention:
The present invention relates to a method of controlling the operation of a pump of a relatively large capacity and, more particularly a method of controlling the operation of a pump in a pumping installation particularly suitable for a variable-speed reversible pump hydraulic turbine in a pumping-up electric power station or the like.
2. Description of the Prior Art:
Generally, in pumping-up electric power stations or the like, hydraulic machinery, operated as a hydraulic turbine in a generating mode and as a pump in a pumping-up mode, is used, and then this machinery is switched to be used so that energy is supplied to an electric power system through a generator in generating mode and consumed through an electric motor in pumping-up mode. In this case, it is known that when the pump is operated, there is a case of illustrating a counterflow characteristic (a characteristic of partially exhibiting positive values for dH/dQ) in its high pump head operation as shown in region a encircled by a broken line in FIG. 3.
Previously, such counterflow pumping characteristic has been dealt with by performing control of providing a sufficient margin particularly in the light of the scale of the net pump head so that the characteristic is overcome, i.e. such operating range of the counterflow characteristic cannot be approached.
Japanese Patent Unexamined Publication No. 62-186069 proposes controlling variable speed pumps wherein, when the load applied on the pump is increased, the driving input is first increased to increase the rotating speed and then the opening of the guide vanes is increased in order that the operating point (on the characteristic of the net pump head vs. the flow quantity) will never fall into the above mentioned range exhibiting the counterflow characteristic on the transient condition. Additionally, it is further proposed that when the load is decreased, the decreasing of the guide vane opening should be completed before the completion of decreasing the rotating speed although these operations are performed simultaneously. Thus, the proposed approach is preferably in overcoming the counterflow characteristic during the operation of only one pump.
However, when operating a pump in a plurality of pumping installations including a plurality of pumps each of which share at least one common pipeline, either upstream or downstream of the pumps, the net pumping head H of one pump rapidly increases by water hammer interference wave transmitted from the other pump through the common pipeline. Although there is a possibility of falling into the above-mentioned counterflow characteristic, the last mentioned proposed arrangement does not address such problems.
Japanese Patent Unexamined Publication No. 61-149583 proposes a method of activating a pump, in which, when the pump is activated, i.e. when the pump is brought from a shut-off state to a full variable-speed pumping state under the desired load, the rotating speed is increased in a stepwise manner in accordance with the gradually increased opening of the guide vanes, and, finally, a proper rotating speed and a proper guide vane opening are achieved under the desired load. However, this last proposed method does not take into account any particular manner of control after a variable-speed operation has been fully initiated. Further, the proposed control does not take into account the water hammer interference transmitted from another pump provided in a common pipeline.
A method of controlling the operation of a variable-speed pumping installation is also proposed in Japanese Patent Unexamined Publication No. 61-175271, in which the pump speed is controlled such that, when the water level reference, i.e. the total difference Hg, between the upper and lower reservoirs is in excess of a predetermined value H.sub.pg, the rotating speed of the pump is corrected by increasing the rotating speed by an amount corresponding to the excess. That is, when H.sub.g &lt;H.sub.pg, the pump is operated at a constant speed such that the rotating speed equal the predetermined value N.sub.0, and when H.sub.G &gt;H.sub.P, the pump is operated at the variable speed such as the rotating speed of N.sub.0 X(H.sub.G /H.sub.PG).sup.1/2 which merely shows an example of the function determining the relation of H.sub.G and N in a stationary state. Therefore, since the method of operating only one pump is disclosed therein, the method has no measures to the water hammer interference of the other pump sharing the common pipe line together with one pump.
Furthermore, Japanese Patent Unexamined Publication No. 53-147145 proposes an arrangement wherein, when a constant speed pump is shifted to the real pumping-up operation by opening a guide vane after priming hydraulic pressure is accomplished, the guide vane is not rapidly opened to a proper opening corresponding to a predetermined value H.sub.G so that the operating point of the pump does not fall into the counterflow characteristic but is kept in the hold state within the sufficient times just before proper opening of the guide vane.
Disadvantages of the prior art proposals reside in the manner of controlling the rotating speed and the guide vane opening to overcome the counterflow characteristics since such control is merely determined on the basis of behavior of only one pump and does not take into consideration problems relating to a pump sharing a common pipeline in the upstream or downstream side of the pump together with other machinery. Moreover, although there is a possibility of shifting an operation point of one pump into the counterflow characteristic due to the water hammer interference generated in the other machinery, the prior art does not provide an effective proposal overcoming the noted problems.
A further disadvantage of the prior art resides in the fact that no means are proposed for preventing the counterflow characteristic and continuing the pump operation when, for some reason, the operation point of the pump falls into the counterflow characteristic.
Furthermore, although the prior art proposals recognize that correctly determining whether or not the operation point of the pump falls into the counterflow characteristic is necessary to prevent damaging of the pump during operation in a counterflow condition, no proposals address the problem or provide a solution thereto.
To understand the counterflow characteristic of a pump, reference is made to FIG. 3 graphically illustrating a characteristic curve of a relationship between a pumped quantity Q and net pump head H of a pump with a guide vane opening maintained constant. On the characteristic curve shown in FIG. 3, the operation point of the pump shifts from a point A.sub..alpha. where the value of the pump head H is H.sub..alpha., to a point A.sub..beta. where the pump head value is H.sub..beta.. Subsequently, a point A.sub.X1 where the value of the head H is H.sub.X has been reached, any increase in the value of the pump head H would result in a direct shift to a point A.sub.X2, causing a sudden decrease of the pumped quantity Q. Subsequently, the pumped quantity Q increases with an increase in the pump head H and a point A.sub..crclbar. is reached, where the pump head value of H.sub..crclbar. is reached.
The depth of the pump in the characteristic curve representing the counterflow, as shown in FIG. 4, will change in dependence upon the guide vane opening. In regions adjacent to such pump, even though this is not presented so distinct, the waterflow is unstable at the pump runner and therefore a similar problem, either serious or negligible, will be present. In FIG. 4 which, like FIG. 3, shows the relationship between the net pump head H and the pumped quantity Q, the curves are plotted with the guide vane opening Y being a parameter, with FIG. 4 showing Y.sub.1 &gt;Y.sub.2 &gt;Y.sub.3 &gt;Y.sub.4. The cause of the counterflow characteristic will be described. That is, when the pump head becomes higher, a radial deviation occurs in the water flow through the pump runner, causing reversed water flows or counterflows 104 and 105, as shown in FIGS. 5 and 6. As shown in FIGS. 5 and 6, a pump includes a pump runner, with the runner including a band segment 102 and a crown portion 103. A vane 107 is provided having vane surfaces 107a, 107b. A counterflow 104 occurs in the runner 101 at the outlet port of the waterflow, and another counterflow 105 occurs at the inlet port.
Under such conditions, it is considered that the water flow may come off the vane surfaces 107A and 107B resulting in a so-called "stalling" state.
Therefore, the above mentioned prior art constructions had the same disadvantages as Japanese Patent Unexamined Publication No 53-147145.